1. Field of the Invention
The present invention relates to reducing NOX emissions, and more particularly, to selective catalytic reduction (SCR) systems for reducing NOX emissions.
2. Description of Related Art
A variety of devices and methods are known in the art for reducing NOX emissions in industrial settings. Of such devices, many are directed to reducing NOX emissions through SCR processes.
The combustion of fossil fuels, such as coal, oil, and industrial or natural gas produces environmentally hazardous substances, including nitric oxide (NO) and nitrogen dioxide (NO2). Nitric oxide and nitrogen dioxide are collectively called nitrogen oxide, or NOX. In the normal combustion process of fossil fuels, the major portion of NOX is NO. The production of NOX can occur when fossil fuel is combusted in a variety of apparatuses, including refinery heaters, gas turbine systems, and boilers, such as in steam plants. The fuel may include coal, oil, gas, waste products, such as municipal solid waste, and a variety of other carbonaceous materials. Federal, state, and regional agencies have established regulations to limit NOX emissions from power plants and other sources.
To comply with governmental regulations, NOX emissions are regulated by combustion controls or utilizing post-combustion methods. The combustion control techniques include boiler tuning, utilization of low NOX burners and/or over-fire air, fuel staging, and other techniques aimed at suppressing NOX formation. These techniques are capable of 25 to 60 percent NOX reduction efficiency. However in many cases, governmental regulations or permits require higher NOX removal efficiency. To accomplish such NOX emissions limits, post-combustion flue gas treatment methods have been commercialized. These methods include selective non-catalytic reduction (SNCR) and selective catalytic reduction (SCR) processes, combinations of the two processes, and other methods. Higher NOX removal efficiencies (80 to over 90 percent) are possible only when utilizing SCR technology.
SCR reactor technology is used to treat exhaust gases from an industrial process, such as energy production, before the gas is released into the atmosphere. The SCR reactor process relies on the use of a catalyst to treat the exhaust gas as the gas passes through the SCR reactor. Both NOX reducing agent and a catalyst reactor are required for the SCR process to proceed. Because the catalyst is an integral part of the chemical reaction, great effort is used to provide maximum exposure of the catalyst to the exhaust gas and to ensure that all the NOX comes sufficiently into contact with the catalyst and the reducing agent for treatment. In this technology, the SCR catalyst is placed in an optimum temperature window of typically between 550 to 750 degrees Fahrenheit. Because the NOX reducing agent is expensive and consumed in large quantities, new challenging problems need to be addressed concerning reagent utilization and its distribution. If the reducing agent (e.g., ammonia) is not entirely consumed in the SCR process, it may be released into the atmosphere, which is referred to as “slip.” Slip increases the cost of the reagent consumption, resulting in non-optimal utilization of the reducing agent and can also cause fouling of downstream equipment. In addition, governmental regulations limit quantities of the allowable release of reagent into the atmosphere. As a result, proper control of the SCR process requires strict control of both NOX and reducing agents released into the atmosphere.
There are a number of known NOX reducing agents. A commonly used NOX reducing agent is ammonia. The principal process for the removal of NOX from the flue gas flow is the injection of a reducing agent, such as ammonia, urea, or any of a number of other known reducing agents, into the flue gas flow. For example, the selective catalytic reduction of NOX involving the injection of ammonia (NH3) into a flue gas flow in the presence of a catalyst occurs as the following chemical reactions:4NO+4NH3+O2→(with catalyst) 4N2+6H2O; (main reaction) and2NO2+4NH3+O2→(with catalyst) 3N2+6H2O.
The main reaction proceeds over a catalyst layer within a temperature range of 600° F. to 750° F. Major components of the catalyst include titanium dioxide (TiO2) and vanadium pentaoxide (V2O5). Additionally, tungsten oxide (WO3) and molybdenum trioxide (MoO3) are added to increase thermal resistance and to limit the deteriorating effects of the catalyst's poisons. Proper selection and sizing of the catalyst volume are critical to achieve the required system performance. Catalyst volume is determined based on catalyst chemical activity, assumed catalyst deactivation rate, deviation of temperature and flue gas flow, and the molar ratio of NH3/NOX across the catalyst bed cross section.
An ammonia injection grid (AIG) is typically utilized to inject vaporized ammonia into the SCR reactor. Because of the desire in the conventional art to inject a homogenous mixture of flue gas and ammonia into the SCR reactor, the ammonia injection grid is usually located immediately “upstream” from the SCR catalyst reactor. In addition to locating the ammonia injection grid immediately before the SCR catalyst reactor, the ammonia injection grid is equipped with jet injectors to further ensure that the ammonia vapor is adequately and evenly distributed across a cross-sectional area, or face, of the catalytic reactor chamber of the SCR system.
U.S. Pat. No. 5,104,629 to Dreschler, U.S. Pat. No. 5,603,909 to Varner et al., U.S. Pat. No. 4,160,805 to Inaba et al., and U.S. Pat. No. 5,988,115 to Anderson et al., all describe various techniques for distributing reagent over a catalyst. However, although the prior art provides SCR system arrangements that are effective for high reduction of NOX concentrations in flue gas, there remain problems with implementing control of NOX emissions without emission of unreacted ammonia. The main problem with the simultaneous control of NOX and NH3 emissions stems from the inability to adjust the ammonia concentration profile to the NOX concentration profile at the face of the SCR catalyst. Disparities between the ammonia concentration profile or the NOX concentration profile lead to reduced NOX efficiency (in the case of insufficient ammonia supply) or to emissions of unreacted ammonia (in the case of oversupply of ammonia). This problem is compounded by the fact that the NOX concentration profile is highly non-uniform across the catalyst face and changes with different operating parameters. Moreover, even with homogenous ammonia vapor injection, the problem of ammonia slip still occurs.
Various solutions to these problems have been suggested, for example in U.S. Patent Publication No. 2004/0057889A1 to Buzanowski (hereinafter “Buzanowski”). Buzanowski describes an ammonia distribution grid that provides control of the adjustment and distribution of ammonia injection and continuously matches the changing NOX concentration profile with an ammonia concentration profile throughout the duct. However, the system described by Buzanowski requires large numbers of ammonia injectors, valves, and sensors in the grid. Moreover, the control of a given ammonia injector in Buzanowski is based on a measurement directly downstream from the given injector, without accounting for the influence of other injectors. However, it is not always the case that each injection nozzle has a clearly defined influence field. There is usually a degree of influence from two or more injection nozzles on ammonia concentration at any single downstream location.
Past research into automatic control of SCRs has focused on transient system response to load changes. Numerous improved control schemes have been proposed to limit ammonia slip during boiler load changes. These include various feed forward strategies, fuzzy logic, and multivariable process control (MPC). Of equal importance to limiting ammonia slip is the uniform distribution of ammonia, NOX, temperature and velocity across the catalyst bed. This uniformity has traditionally been achieved by careful design of the reactor, ductwork, and flue gas mixing system with the use of physical models. Ammonia distribution is then manually adjusted in the field to get to the best possible ammonia-to-NOX ratio across the catalyst bed. Physical constraints of the plant and flue gas pressure drop, however, limit the extent to which this uniformity can be achieved.
Such conventional methods and systems generally have been considered satisfactory for their intended purpose. However, there still remains a continued need in the art for improved automatic distribution of flow of reagents over catalysts in SCR systems. There also remains a need in the art for such a method and system that are inexpensive and easy to make and use. The present invention provides a solution for these problems.